Raffinose family oligosaccharides are derivatives of sucrose, which are represented by o-α-D-galactopyranosyl-(1→6) n-o-α-D-glucopyranosyl-(1→2)-β-D-fluctofuranoside as the general formula, and they are designated “raffinose” when n=1, “stachyose” when n=2, “verbascose” when n=3, and “ajugose” when n=4.
The greatest contents of such raffinose family oligosaccharides are found in plants, except for sucrose, and it has been known that they are contained not only in higher plants including gymnosperms such as pinaceous plants (e.g., spruce) and angiosperms such as leguminous plants (e.g., soybean, kidney bean), brassicaceous plants (e.g., rape), chenopodiaceous plants (e.g., sugar beet), malvaceous plants (e.g., cotton), and salicaceous plants (e.g., poplar), but also in green algae, chlorella. Thus, they occur widely in the plant kingdom similarly to sucrose.
Raffinose family oligosaccharides play a role as reserve sugars in the storage organs or seeds of many plants or as translocating sugars in the phenomenon of sugar transportation between the tissues of some plants.
Furthermore, it has been known that raffinose family oligosaccharides have an effect of giving good conditions of enterobacterial flora, if present at a suitable amount in food. Therefore, raffinose family oligosaccharides have already been used as a functional food material for addition to some kinds of food and utilized in the field of specified healthful food.
Raffinose family oligosaccharides having such a role and utility are produced by the raffinose oligosaccharide synthesis system beginning with sucrose in many plants. This biosynthesis system usually involves a reaction for the sequential addition of galactosyl groups from galactotinol through an α(1→6) bond to a hydroxyl group attached to the carbon atom at position 6 of a D-glucose residue in a sucrose molecule.
In the first step of this biosynthesis system, raffinose synthase is an enzyme concerned in the reaction of raffinose production by combining a D-galactosyl group from galactotinol through an α(1→6) bond with a hydroxyl group attached to the carbon atom at position 6 of a D-glucose residue in a sucrose molecule. It has been suggested that this enzyme constitutes a rate limiting step in the above synthesis system, and it has been revealed that this enzyme is quite important in the control of biosynthesis of raffinose family oligosaccharides.
The control of expression level or activity of raffinose synthase in plants makes it possible to change the contents of raffinose family oligosaccharides in these plants. However, raffinose synthase, although the presence of this enzyme itself was already confirmed in many plants by the measurement of its activity with a biochemical technique, has not yet been successfully isolated and purified as a homogeneous protein. In addition, the amino acid sequence of this enzyme is still unknown, and no report has been made on an attempt at beginning to isolate a gene coding for this enzyme.